Secondary batteries, including a lithium ion secondary battery, lithium ion polymer battery and a super capacitor (electrical double layer capacitor and pseudo-capacitor), have undergone a change in market demand into development of a battery having low weight, high energy density and high capacity, as the consumer's need has been changed by digitalization and high performance conversion of electronic appliances. In addition, development of hybrid electric vehicles, electric vehicles and fuel cell vehicles have been conducted actively to handle the future energy and environment problems, and thus scale-up of batteries as power supply sources for vehicles has been required.
Secondary batteries are those capable of being charged and discharged and may be classified into nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries, lithium (Li) secondary batteries, or the like. Among those, lithium secondary batteries are significantly advantageous in that they have a higher driving voltage and energy density per unit weight as compared to the other secondary batteries.
In general, a secondary battery has a structure in which a separator is inserted between a cathode and an anode to form an electrode assembly, an electrolyte is injected thereto and the resultant electrode assembly is sealed in a casing. The electrode assembly may be classified into a jelly-roll type electrode assembly formed by applying an electrode active material onto both surfaces of elongated sheet-like collector foil to form a cathode and an anode, inserting a separator between the cathode and anode, and winding the resultant electrode assembly; and a stack type electrode assembly formed by applying an electrode active material onto both surfaces of collector foil having a predetermined unit size and stacking multiple cathodes and anodes successively with a separator inserted therebetween. Secondary batteries may also be classified, depending on the shape of an electrode assembly enclosed in a battery casing, into a cylindrical battery including an electrode assembly enclosed in a cylindrical metallic can, a prismatic battery including an electrode assembly enclosed in a prismatic metallic can and a pouch type battery including an electrode assembly enclosed in a pouch-like casing made of an aluminum laminate sheet.
Meanwhile, lithium secondary batteries forming a main part of secondary batteries are disadvantageous in that they have low safety. For example, when a battery is overcharged to about 4.5V or higher, a cathode active material may be decomposed, lithium metal dendrite may grow on an anode and decomposition of an electrolyte may occur. During this, heat emission occurs to accelerate the above-mentioned decomposition reactions and several side reactions, resulting in ignition and explosion of the battery. In addition, when a lithium secondary battery is charged first after being sealed, a large amount of gas is generated inside of the battery. Such gas generation causes generation of bubbles between an electrode and a polymer electrolyte layer, resulting in rapid degradation of the quality of a battery due to contact inferiority.
Therefore, some attempts have been made to solve the above-mentioned problems by adding an additive to an electrolyte or providing a battery with a device, such as a current interruptive device (CID), to interrupt electric current and to release internal pressure upon the abnormal operation of a secondary battery. However, such an additive adversely affects the quality of a secondary battery and the device, such as CID, causes an increase in manufacturing cost of a secondary battery and makes the manufacturing process complicated.
Under these circumstances, there is a need for developing a pouch-type secondary battery which can improve the safety upon overcharge.